1. Field of The Invention
The present invention relates to a liquid crystal display device (LCD), and more particularly, to an array substrate for an LCD, a method of fabricating the array substrate, and a repairing method of the array substrate.
2. Discussion of The Related Art
Generally, an LCD device uses optical anisotropy and polarization properties of liquid crystal molecules to display an image. The liquid crystal molecules have an alignment direction along their thin and long shapes. The alignment direction of the liquid crystal molecules can be controlled by applying an electric field to the liquid crystal molecules. In other words, as the intensity of the electric field is changed, the orientation of the alignment direction for the liquid crystal molecules also changes. Since incident light through liquid crystal molecules is refracted based on the orientation of the liquid crystal molecules, due to the optical anisotropy of the aligned liquid crystal molecules, intensity of the incident light can be controlled such that images can be displayed.
Among the various types of LCD devices commonly used, active matrix LCD (AM-LCD) devices having thin film transistors (TFTs) with pixel electrodes connected to the TFTs disposed in matrix form have high resolution and superiority in displaying moving images. FIG. 1 is a schematic perspective view of an active matrix liquid crystal display device according to the related art. As shown in FIG. 1, an LCD device includes a first substrate 80 having a transparent common electrode 92 on a color filter layer 89 including red, green and blue sub-color filters 89a to 89c and a black matrix 85 between the adjacent red, green and blue sub-color filters 89a to 89c, and a second substrate 10 having a pixel electrode 60, a switching element “Tr” and array lines. Further, a layer of liquid crystal molecules 70 is interposed between the first and second substrates 80 and 10. The first and second substrates 80 and 10 are commonly referred to as a color filter substrate and an array substrate, respectively. The switching element “Tr,” for example, is a thin film transistor (TFT) disposed in a matrix arrangement and connected to a gate line 13 and a data line 30 crossing each other. A pixel region “P” is defined at a crossing portion of the gate line 13 and the data line 30. The pixel electrode 60 is made of a transparent conductive material disposed in the pixel region “P.”
The LCD device is driven with an electro-optical effect on the liquid crystal molecules 70. Since the liquid crystal molecules 70 have dielectric anisotropy and spontaneous polarization, a dipole is formed in the layer of liquid crystal molecules 70 when a voltage is applied across the layer of liquid crystal molecules 70. Thus, an alignment direction of liquid crystal molecules changes according to the direction and the intensity of an electric field resulting from the applied voltage. Optical properties of the layer of liquid crystal molecules 70 depends on the alignment state of the liquid crystal molecules 70 so as to in a effect be a kind of electrical light modulator. Therefore, the LCD device displays images by blocking or transmitting light using the layer of liquid crystal molecules 70 as an electrical light modulator.
Although not shown, first and second polarizers, which transmit light parallel to polarization axis, are disposed on outer sides of both the first and second substrates 80 and 10, respectively. A backlight unit (not shown) is disposed under the one of the polarizers as a light source.
FIG. 2 is a schematic plan view showing an array substrate for an active matrix LCD device according to the related art. As shown in FIG. 2, a gate line 13 and a data line 30 cross each other to define a pixel region “P,” and a TFT “Tr” is disposed at a crossing of the gate line 13 and the data line 30. A scan signal and an image signal are supplied to the gate line 13 and the data line 30 from an external circuit (not shown), respectively. The switching element TFT “Tr” is connected to the gate line 13, the data line 30, and a pixel electrode 60 in the pixel region
The TFT “Tr” includes a gate electrode 15, an active layer 23, and source and drain electrodes 33 and 36. The gate electrode 15 is connected to the gate line 13. The source and drain electrodes 33 and 36 are formed to overlap the gate electrode 15 and are spaced apart from each other on the active layer 23. The active layer 23 may be formed of one of amorphous silicon (a-Si:H) and polycrystalline silicon. For example, the active layer 23 in FIG. 2 can be made of amorphous silicon. The source electrode 33 is connected to the data line 30 and the drain electrode 36 is connected to the pixel electrode 60 in the pixel region “P.” Although not shown, gate and data pads are at end portions of the gate and data lines 13 and 30, respectively.
A first storage electrode 14 occupies a portion of the gate line 13 of an adjacent pixel and a second storage electrode 31 is disposed over the first storage electrode 14. The pixel electrode 60 extends over the second storage electrode 31 and is connected to the second storage electrode 31 via a storage contact hole 49. The first and second storage electrodes 14 and 31 together with an intervening insulating layer (not shown) constitute a storage capacitor “CST” for maintaining an applied voltage until next signal is applied to the pixel electrode 60.
When static electricity occurs or a foreign material adheres to the array elements, such as the TFT “Tr”, the gate line 13 or the data line 30, signals cannot be normally applied to the gate line 13 and the data line 30. Therefore, the TFT “Tr” may not be normally turned ON/OFF, so that the LCD device has dead pixels that can be recognized by users as a point defect, such as a bright point or a dark point. Too many dead pixels make a bad product that can not be sold. The LCD device has at least several thousand pixels to several million pixels. Consequently, a small amount of dead pixels can occur and the LCD device can still be seen as a good product. To reduce the appearance of dead pixels, repairs can be made.
In a normally white mode LCD device, when a voltage is not applied to the pixel, the pixel is in a white state. In contrast, when the voltage is applied to the pixel of a normally white mode LCD device, brightness of the pixel is controlled in accordance with the intensity of the applied voltage by controlling transmitted light. For example, when a voltage with a maximum value is applied to the pixel, the pixel is in a black state due to blocking light transmission. When the point defect of a bright point occurs due to a dead pixel, it is easily recognized by users.
A repair process should be at least be performed on a bright point so that the dead pixels can be maintained as a dark point to reduce recognition of the dead pixel by users. A bright point can be repaired by repeatedly applying a gate voltage to the dead pixel because the layer of liquid crystal molecules in a pixel inherently acts a capacitor so that the gate voltage is maintained until the next application of the gate voltage. When the point defect is a dark point, a repair process is not needed as much for a bright point because a dark point is not as noticeable to a user.
FIG. 3 is a schematic plan view showing a repairing process of the array substrate for the LCD of FIG. 2 according to the related art. As shown in FIG. 3, when the pixel “P” of a normally white mode LCD device is determined to be dead, the drain electrode 36 of the dead pixel “P” is cut using a laser. Next, the pixel electrode 60 is connected to the first storage electrode 14 through the storage contact hole 49 using a laser. Accordingly, the repaired pixel electrode 60 receives the gate voltage applied to the gate line 13 of an adjacent pixel through the first storage electrode 14. Therefore, since a gate voltage is repeatedly applied to the repaired pixel electrode 60 that is disconnected from the transistor Tr, the pixel is changed into a dark point and the inherent capacitance of the pixel maintains the dark point until the gate voltage is applied again.
When the LCD device is a normally black mode, the dead pixel may be repaired by cutting the drain electrode 36 of the pixel without connecting the pixel electrode 60 to the first storage electrode 14. Because an LCD with good image quality is in demand, a method for repairing both dark spots and bright spots is needed. More specifically, a method for restoring both dark spots and bright spots into active pixels is needed.